Failure to go into gears is a mechanical issue. Could be a fluid pressure problem, could be a clutch/brake problem, or could be a valve body problem. Find yourself a copy of the ASTG service manual for the 722.3/4 (it's available online if you Google for it) and check pressures to make sure you don't have pressure problems. You have confirmed that you have adequate fluid in the system? Engine running, ran through gears pausing in each one, then checked level with the engine still running in park? Won't be as accurate as checking at operating temp, but you should at least be between the marks on the dipstick.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]